The present invention relates to an operational amplifier, and more particularly, to a method for canceling offset voltage of an operational amplifier.
A plurality of operational amplifiers for amplifying analog signals or differential signals are incorporated in an LSI as its basic circuits. The output signal of an operational amplifier includes an error (offset voltage) resulting from characteristic differences between transistors. With such an error, an operational amplifier cannot produce an output signal of 0 V even if the input signal is 0 V. Such an error in the output signal with respect to the input signal is referred to as an offset voltage. As shown in FIG. 1, the offset voltage is expressed as a voltage supply 2, which is connected to an input terminal of an operational amplifier 1.
When an input voltage Vin is applied, an output voltage Vo of the operational amplifier 1 is determined by the value of an input resistor R1 and the value of a feedback resistor R2. The output voltage Vo is calculated using the expression:Vo=(1+R2/R1)×Vin.
In reality, however, the voltage supply 2 shown in FIG. 1 adds an offset voltage e1 to the input voltage Vin. Thus, the output voltage Vo is set as:Vo=(1+R2/R1)×(Vin−e1).
In this way, the offset voltage e1 is also amplified and included in the output voltage Vo. When the input voltage Vin is small, the influence of the offset voltage e1 on the output voltage Vo increases.
A method for canceling such offset voltage of an operational amplifier has been proposed. In this method, an output voltage generated by short-circuiting two input terminals of the operational amplifier is held as an offset voltage, and the held offset voltage is fed back to the input of the operational amplifier.
In one example, Japanese Laid-Open Patent Publication No. 8-18353 describes an operational amplifier including a main amplifier, an auxiliary amplifier, and a holding means. The auxiliary amplifier receives an output signal of the main amplifier and amplifies the signal in a direction opposite to the output signal. The holding means holds an output signal of the auxiliary amplifier and feeds back the held output signal to the main amplifier. In this operational amplifier, the offset voltage of the main amplifier is reduced to a fraction of the gain of the main amplifier.
As another example, Japanese Laid-Open Patent Publication No. 2001-292041 describes an operational amplifier. In the operational amplifier, voltage including an offset and amplified by an operational amplifier circuit is accumulated in a capacitor device. A voltage value of the operational amplifier circuit is feedback-controlled based on the accumulated voltage.